1. Field of the Invention
The present invention relates to extracting method and apparatus of a blood vessel crossing/branching portion and, more particularly, to extracting method and apparatus of a blood vessel crossing/branching portion of a fundus image of the eye.
2. Description of the Related Art
In association with the recent aging society, an early diagnosis of a blood vessel disorder is becoming important in order to improve a QOL of the nation. In order to perform the early diagnosis of the blood vessel disorder, it is desirable that the blood vessel can be easily observed and a fundus photograph of the eye is useful for this purpose.
In a medical treatment using the fundus photograph, information which is clinically important is obtained by observing, particularly, the blood vessel crossing/branching portion of the fundus of the eye. For example, information of imperforation and stricture of the blood vessel crossing portion due to arterial sclerosis or the like is obtained from the blood vessel crossing portion. Information which is important in a blood flow evaluation is obtained from the blood vessel branching portion by, particularly, an observation by a fluorescence fundus photograph of the eye.
Further, when a plurality of fundus images are compared and their aging changes are evaluated, even in the case of accurately position-matching the fundus images to be compared, by extracting the blood vessel crossing/branching portion as a characteristics point, the position matching can be easily performed. Therefore, a technique for automatically extracting the blood vessel crossing/branching portion from the fundus images has been developed.
As a main technique, there is such a technique that a blood vessel is extracted by binarizing a fundus image, the extracted blood vessel is converted into a thin line, and a crossing/branching portion is obtained. However, according to such a technique, there is a problem in which as for the fundus image, since a contrast is low and continuity on the image of the blood vessel is not guaranteed, the extraction itself of the blood vessel is relatively difficult, so that the blood vessel crossing/branching portion cannot be correctly extracted.
Therefore, in the Official Gazette of Japanese Patent Application Laid-Open No. 2001-070247, there is disclosed a method whereby a candidate region is selected from a fundus image and the presence or absence of a blood vessel crossing in the candidate region is detected under such conditions that, in the candidate region, four or more blood vessels pass through its outer circumferential portion and that the blood vessel runs in a center portion of the candidate region. According to such a method, the candidate region is divided into a plurality of square small regions and an average value of pixel values of each small region existing in the outer circumferential portion among them is calculated. Further, a difference between the average values of the adjacent small regions is calculated and a threshold value process is executed, thereby discriminating whether or not the blood vessel passes through the small region. After that, the pixel value of the small region in the outer circumferential portion which has been determined that the blood vessel passes and the pixel value of the small region in the center portion are compared, thereby discriminating whether or not the blood vessel passes through the small region in the center portion.
The technique disclosed in Japanese Patent Application Laid-Open No. 2001-070247 is a useful technique in the case of extracting the blood vessel crossing/branching portion from the fundus image from which it is difficult to extract the blood vessel. However, according to such a technique, if there is a blood vessel running in the center portion of the candidate region and another one blood vessel runs in the candidate region, even when those two blood vessels do not cross, it is erroneously determined that there is a crossing. On the other hand, even when there is a crossing, if at least blood vessel runs in the center portion of the candidate region, an image of such a region is extracted. There is consequently such a problem that an image of a region where a crossing portion does not exist at the center in the candidate region and it is difficult to observe the crossing portion which is important on diagnosis is also extracted.